As the withstand requirements and interruption requirements of circuit breakers increase, circuit breakers must physically withstand the forces generated by the higher current. Additionally, the higher currents create a magnetic flux which may affect the operation of the circuit breaker. With advances in technology circuit breakers are designed in smaller housings, thus adding to the problem of the undesirable effects of magnetic flux. In particular, where a circuit breaker has a high withstand requirement and the phase conductor is located close to the trip solenoid assembly, nuisance tripping can occur.
A trip solenoid assembly often includes a housing, plunger, spring, permanent magnet and trip coil. The permanent magnet and spring generally exert opposing forces on the plunger. As the trip coil is energized, it generates a sufficient flux opposing the permanent magnet to allow the spring to move the plunger away from the permanent magnet.
When the trip solenoid assembly is positioned in relatively close proximity to a phase conductor, a high current through the phase conductor will create a flux sufficient to upset the balance between the other forces acting on the plunger. If the current through the phase conductor is high, for example, at current levels at or near a withstand rating, the plunger may move to the tripped position even though the trip coil has not been energized. It is desirable for the plunger to remain in its latched position until the electronic trip unit has issued a trip signal.
In the prior art this type of nuisance tripping was avoided by positioning the trip solenoid assembly a greater distance away from the phase conductor or by increasing the margin between the permanent magnet force and the spring force. The latter has the disadvantage of requiring increased energy to unlatch the trip solenoid and circuit breaker operating mechanism. In the prior art this type of nuisance tripping was not as great a concern with those circuit breakers that did not have withstand capabilities.
There is a need for a circuit breaker having a trip solenoid assembly that can accommodate a large current through a nearby phase conductor without nuisance tripping while requiring only a low energy power source for unlatching the operating mechanism.